Method of alloying an electrode to a germanium semi-conductive body



United States Patent METHOD OF ALLOYING AN ELECTRODE TO A GERMANIUMSEMI-CONDUCTIVE BODY No Drawing. Application July 20, 1956 Serial No.598,999

Claims priority, application Netherlands July 21, 1955 9 Claims. (Cl.148-15) This invention relates to methods of manufacturing alloyelectrodes on semi-conductive bodies of germanium.

The term alloy electrode is to be understood in this case to mean anelectrode obtained by providing on the body by melting or fusion acertain amount of a suitable element or a suitable alloy, followed bycooling. Such semi-conductive bodies having one or more alloy electrodesare frequently used in crystal diodes, transistors and othersemi-conductive devices. It may be assumed that, during melting, part ofthe semi-conductive material dissolves in the melt and, during cooling,recrystallizes and grows on the original crystal lattice, but now with acertain content of the element or one of the elements from the melt. Thematerial grown may thus have a conductivity-type which differs from thatof the initial material. The element or the alloy is so chosen that theelectrode acquires the desired electrical properties. In most cases, theelectrode must be of the rectifying type, the element or the alloy thenbeing required to have donor or acceptor properties depending uponWhether the germanium is of the p-type or the n-type. In the first case,the alloy may consist of or contain antimony or arsenic; in the lattercase, the alloy may consists of or contain indium, or gallium.

It has previously been suggested to utilize fusing agents for themanufacture of such alloy electrodes, but such agents or fluxes have thedisadvantage that residues or disintegration products thereof lead tolarge leakage currents with rectifying electrodes.

The object of the invention is inter alia to provide a method ofmanufacturing such electrodes, in which said disadvantages do not occur.

According to the invention, the alloy electrode is first applied byfusion at a temperature below 450 C. and then subjected to afterheatingat a temperature higher than the fusion temperature and at any ratehigher than 350 C., the fusing agent or flux used during the firstheating being a substance which may constitute the halide of the elementor alloy applied by fusion and which has a vapour tension or pressure ofat least 10 mms. mercury pressure at the temperature of the afterheatingprocess.

When use is made of this method, fusing agents which may satisfactorilybe used are the halogens themselves, the halogen-hydrogen acids, severalhalides having a high vapour tension and thehalogen-hydrogen-moleculecomplexes of organic compounds of the halides,in particular the halides of the donors or acceptors present in theelectrodes can be used very well.

The invention will now be explained with reference to some examples forthe manufacture of transistors.

Example I A small ball of indium having a weight of 3.5 mgs. is laid ona thin disc of germanium of the H-COHdUClIlVilEY type having a thicknessof 150 microns. The whole is heated in a tube up to a temperature of 300C. while passing dry hydrogen through the tube. Melting together thendoes not take place or substantially does not take place. Now, forseveral seconds, a little hydrochloric acid, as a flux, is added to thegas passed through, the quantity of which is not critical. Immediatelythereafter, wetting occurs and the ball of indium and the germanium melttogether. However, on account of the low temperature, the germaniumsubstantially does not dissolve in the indium, and thus no penetrationof the liquid-solid interface occurs. In a similar manner, a secondsmall ball of indium having a weight of 0.8 mg. is secured by melting tothe other side of the germanium disc. At the same time, a base contactis secured by soldering with the use of tin.

Subsequently, nitrogen with a trace of oxygen (which serves to preventthe flowing out of the indium) is passed through the tube and thetemperature is increased to 600 C. for a few seconds or longer. The acidresidue, for example traces of indium-chloride, evaporate completelyduring this, or the preceding treatment and are carried away completelyby the How of nitrogen, while the indium alloys further with thegermanium and the alloy now penetrates the disc to form a junction atthe desired depth.

The halogens themselves or other halogen-hydrogen acids may be used as afusing agent, by applying them in gaseous form, the same as has beendescribed in relation to hydrochloric-acid. If, however, the halogenswould react violently with the hydrogen, it is preferred to use nitrogenas a carrier.

Many other metals and alloys may be fused on germanium in the same wayas described here with regard to indium. So for instance lead containing2-10 percent (by weight) of antimony, bismuth containing 2% of gallium,tin containing 52% of indium, tin containing 510% antimony or 1%arsenic, and lead containing 24% tin plus 1% antimony. Some of theelectrodes made from these alloys are used only as ohmic contacts.

Example I] A small ball consisting of indium is moistened with asaturated solution of pyridine-hydrochloride in water. It is laid on athin disc of germanium and heated in a tube to approximately 300 C. for2-5 minutes while passing through dry hydrogen. The indium now readilyflows out on the germanium while obtaining very good adhesion to definethe ultimate alloying area, but no penetration of the alloy frontoccurs. In a similar manner, a second small ball of indium is providedby fusion on the other side, at the same time a base contact beingsecured by soldering with the use of tin. Finally, an afterheatingtreatment is carried out as in the first example to cause the desiredextent of penetration of the alloy front.

For moistening, use may also be made of otherhalogen-hydrogen-molecule-complexes of organic compounds, such asaniline-chlorohydrate, dimethylaniline-chlorhydride andguanidine-chlorhydride. Good results have also been obtained withsolutions of indium chloride, indium bromide and indium iodide in water,notwithstanding the hydrolisation of such solutions.

Example [[1 Germanium pellets on which small balls of indium have beenlaid are heated in a tube at about 350 C. in a flow of a mixture ofhydrogen and nitrogen. As soon as the indium has been molten, somenitrogen containing boron chloride is passed over the pellets causinggood adherence of the indium to the germanium. The treatment of heatingin the mixture of hydrogen and nitrogen Patented May 19, 1959' It 'willbe evident that it is advantageous to utilize a fusing agent or fluxhaving a vapour pressure which is as high as possible within practicallimits. It is preferable to ensure that the electrode applied by fusiondoes not contain elements leaving residues which are difficult toevaporate with the fusing agent. If a residue should remain, it must atany rate be removed during the etching process which usually follows thefusion process.

What is claimed is:

l. A method of alloying an electrode to a germanium semi-conductivebody, which comprises placing an electrode-forming material inengagement with said germanium body, preheating said body andelectrode-forming material in the presence of a gaseous or liquid fluxand at a'first temperature below 450 C. and for 'a predetermined timeinterval at which the electrode-forming material fuses and adheres tobut does not substan tially penetrate the germanium body thereby todefine the alloying area, thereafter reheating the body and ma terial ata second temperature above 350 C. and above the first temperature and atwhichthe said material substantially penetrates the germanium body tothe required degree, said flux being constituted of areadily-volatilizable halogen-ion-donating materialhaving a vaporpressure at said second temperature'of at least mms. of Hg and reacting"with said body and material to produce only readily-volatilizableresidues, whereby detrimental residues of said flux on the germaniumbody are avoided.

2. A method as set forth in claim 1 wherein the flux is a halide of anelement of the electrode-forming material.

3. A method as set forth in claim 2 wherein the electrode-formingmaterial is constituted principally of indium, and the flux is indiumchloride.

4. A method as set forth in claim 1 wherein the flux is a halogen.

5. A method as set forth in claim 1 wherein the flux is ahalogen-hydrogen-molecule-complex of an organic compound.

6. A method as set is a halide.

7. A method of making an alloy electrode :to a germanium semiconductivebody, comprising the steps of heating the body with electrode materialin juxtaposed forth in claim 1 wherein the flux relationship at atemperature below 450 C. in a substantially non-oxidizing atmosphere,adding to said at mosphere a small amount of hydrochloric acid as a fluxto cause the electrode material and body to wet one another and fusewithout any substantial penetration occurring, thereafter heatingthebody and material at a second higher temperature, above 350 C., tocause the required degree of penetration to ocur.

8. A method of alloying an electrode to a germanium semiconductive body,which comprises placing an electrode-forming material in engagement withsaid germanium body, preheating said body and electrode-formingmaterialin the presence of a liquid flux and at a first temperaturebelow 450 C. and for a predetermined time interval at which theelectrode-forming material fuses and adheres to but does notsubstantially penetrate the germanium body thereby to define thealloying area, thereafter reheating the body and material at a secondtemperature above 350 C. and above the first temperature and at whichthe said material substantially penetrates the germanium body to therequired degree, said liquid flux being constituted of areadily-volatilizable halogen-hydrogen acid having a vapor pressure atsaid second temperature of at least 10 mms. of Hg and reacting with saidbody and material to produce only readily-volatilizable residues,whereby detrimental residues of said flux on the germanium body areavoided.

9. .A method of alloying an electrode to a germanium semi-conductivebody, Which comprises placing an electrode-forming material inengagement with said germanium body, preheating said body andelectrode-forming material in the presence of a flux consistingessentially of hydrochloric acid and at a first temperature below 450 C.and for a predetermined time interval at which the electrode formingmaterial fuses and adheres to but does not substantially penetrate thegermanium body thereby to define the alloying area, thereafter reheatingthe body and material at a second temperature above 350 C. and above thefirst temperature and at which the said material substantiallypenetrates the germanium body to the required degree, said flux reactingwith said body and material to produce only readily-volatilizableresidues, whereby detrimental residues of said flux on the germaniumbody are avoided.

References Cited in the file of this patent UNITED STATES PATENTS

1. A METHOD OF ALLOYING AN ELECTRODE TO A GERMANIUM SEMI-CONDUCTIVEBODY, WHICH COMPRISES PLACING AN ELECTRODE-FORMING MATERIAL INENGAGEMENT WITH SAID GERMANIUM BODY, PREHEATING SAID BODY ANDELECTRODE-FORMING MATERIAL IN THE PRESENCE OF A GASEOUS OR LIQUID FLUXAND AT A FIRST TEMPERATURE BELOW 450*C. AND FOR A PREDETERMINED TIMEINTERVAL AT WHICH THE ELECTRODE-FORMING MATERIAL FUSES AND ADHERES TOBUT DOES NOT SUBSTANTIALLY PENETRATE THE GERMANIUM BODY THEREBY TODEFINE THE ALLOYING AREA, THEREAFTER REHEATING THE BODY AND MATERIAL ATA SECOND TEMPERATURE ABOVE 350*C. AND ABOVE THE FIRST TEMPERATURE AND ATWHICH THE SAID MATERIAL SUBSTANTIALLY PENETRATES THE GERMANIUM BODY TOTHE REQUIRED DEGREE, SAID FLUX BEING CONSTITUTED OF AREADILY-VOLATILIZABLE HALOGEN-ION-DONATING MATERIAL HAVING A VAPORPRESSURE AT SAID SECOND TEMPERATURE OF AT LEAST 10 MMS. OF HG ANDREACTING WITH SAID BODY AND MATERIAL TO PRODUCE ONLYREADILY-VOLATILIZABLE RESIDUES, WHEREBY DETRIMENTAL RESIDUES OF SAIDFLUX ON THE GERMANIUM BODY ARE AVOIDED.